The purpose of this project is to study the mechanism of retroviral induced aplastic anemia and to ascertain if some cases of human aplastic anemia are associated with a persistent retroviral infection. Though feline leukemia virus (FeLV) is well known to cause leukemias in cats, it actually causes more cases of erythroid hypoplasia and aplastic anemia. Therefore, the pathogenesis of FeLV induced aplastic anemia will be studied in order to gain insights into the development and etiology of human aplastic anemia. In addition cells from the bone marrow and peripheral blood from patients with aplastic anemia will be grown in culture in an attempt to induce retroviral growth.